Actually some fantails can get as large as 10 inches (not including tail). My largest is 6 inches without the tail, but with it he's close to 11 inches long. Fantails along with oranda tend to be the larger fancy breeds. Ryukin and ranchu tend to stay smaller (unless they are show fish). The widest goldfish are probably the pearlscales. Some can be as wide as a softball.

Take a look at these pearlscales owned by some of my goldfish-keeping friends:

Well now knowing they are that big (up to six times the length of a betta, not to mention girth!) I would feel bad giving them any less than 48 inches of swimming length! And even then I would feel that I'm pushing it! What large fish!

Pearlscales are chubby and wiggly and have stolen the hearts of many a goldfish-keeper. They are really cute to watch.

My goldfish use the entire tank for swimming. Looking at them and then looking at my 29 gal, I can't imagine keeping them in anything like that. I'm actually looking to upgrade them to a 75 gal in the future.